In PIV the distance which the tracer particles have covered between the illuminations of the interrogation volume is used as a measure of the velocities of the tracer particles in the interrogation volume. By this a double problem is caused. At first, the sign of the flow velocities of the single tracer particles does not result from the multiple exposed pictures of the interrogation volume without some qualification. At second, the velocity components perpendicular to the main extension directions of the interrogation volume which is the so called out-of-plane component can also not be determined without some qualification.
In a known PIV method the interrogation volume is projected stereoscopically on two photo sensors, i. e. in two different projection directions, for recording a double exposed picture of the interrogation volume with each photo sensor. Both double exposed pictures together cover the flow velocities of the tracer particles in all three dimensions. In the method, the determination of the sign of the flow velocity is enabled in that between the two illuminations the interrogation volume is shifted relative to the photo sensors in such a way that a particle image shift on the photo sensors bigger than the maximum particle image shift corresponding to the flow is caused. By this way particle image shifts of only one sign necessarily result in the double exposed pictures of the interrogation volume, and both the magnitude and the sign of the flow velocities can be determined by forming a difference to the artificial shift of the interrogation volume relative to the photo sensors. Such a known PIV method is disclosed in German Patent 42 37 440, for example. This method requires a considerable apparatus expenditure for implementation. The projection optic must comprise two objectives arranged at an angle and two particle image shifting means corresponding to each other, i. e. synchronized with each other. Nevertheless, the resolution of the flow velocities perpendicular to the main extension directions of the interrogation volume is low, because only a comparatively small angle between both projection means of the stereoscopic arrangement can be selected.
In a further known PIV method the interrogation volume is repeatedly illuminated and recorded holographically. For this a photo sensor in the form of a special film is used. However, films usable for taking holographic pictures have the drawback of a low photosensivity. So only the capture of small survey fields and of low flow velocities is possible. Additionally, in a holographic technique an artificial shift of the interrogation volume relative to the photo sensor between the two illuminations of the interrogation volume can also not be avoided, if the sign of the flow velocities of the tracer particles is to be determined. This results in a considerable apparatus expenditure besides the holographic projection arrangement.
More particularly the invention is related to a method known as digital particle image velocimetry abbreviated as DPIV.
Normally in DPIV two pictures of the same interrogation volume are recorded separately and the pictures are evaluated statistically. Liepmann et al. teach in "The role of streamwise vorticity in the near field entrainment of round jets" (Journal of Fluid Mechanics, Vol. 245 (1992), pp. 643-668) to calculate a cross-correlation function between the two pictures recorded at an interval of time and to interpret the peak height of the cross-correlation peak as a qualitative measure for the out-of-plane component of the flow.